1. Field of the Invention
The present invention relates to a semiconductor producing method of forming a highly uniform crystalline silicon film in processes relating to manufacture of insulated-gate semiconductor devices such as thin-film transistors (TFTs) which are formed by using a non-single-crystal, crystalline silicon film provided on a glass substrate, and other semiconductor devices. In particular, the invention is effective in forming a semiconductor device on a glass substrate.
2. Description of the Related Art
In recent years, insulated-gate field-effect transistors having a thin-film active layer (or active region) on an insulative substrate, i.e., thin-film transistors (TFTs), have been studied eagerly.
The TFTs are classified into an amorphous silicon TFT, a crystalline silicon TFT, etc. by the semiconductor material used and its crystal state. The "crystalline silicon" does not always mean single crystal silicon but may mean non-single crystal silicon in some cases. TFTs using the latter are generally called non-single-crystal silicon TFTs.
In general, amorphous semiconductors have a small electric field mobility, which therefore cannot be used for TFTs that are required to operate at high speed. Further, amorphous silicon can provide only a very small P-type electric field mobility, it does not allow formation of a P-channel TFT (i.e., PMOS TFT), so that a complementary MOS (CMOS) circuit cannot be formed by combining P-channel TFTs and N-channel TFTs (NMOS TFTs).
In contrast, since crystalline semiconductors have larger electric field mobilities than amorphous semiconductors, they allow high-speed operation of TFTs. Further, allowing formation of not only an NMOS TFT but also a PMOS TFT, crystalline silicon enables formation of a CMOS circuit.
A non-single-crystal silicon film is obtained by forming an amorphous silicon film by vapor-phase growth and then thermally annealing it for a long time at a proper temperature (usually higher than 600.degree. C.) or irradiating it with strong light such as laser light (optical annealing).
However, where a glass substrate, which is inexpensive and highly workable, is used as an insulative substrate, it is very difficult to form, only by thermal annealing, a crystalline silicon film having a sufficiently large electric field mobility (to allow formation of a CMOS circuit). This is because a glass substrate generally has a low strain temperature (about 600.degree. C.) and therefore it is distorted when its temperature is increased to a value that is necessary to form a crystalline silicon film having a sufficiently high mobility.
On the other hand, where optical annealing is used to crystalline a silicon film formed on a glass substrate, high energy can be applied to only the silicon film without much increasing the temperature of the substrate. The optical annealing is thus very effective for crystallization of a silicon film formed on a glass substrate.
At present, large output pulsed lasers such as excimer lasers are considered most suitable for a light source for optical annealing. Since those lasers have much larger maximum energies than CW lasers such as an argon ion laser, they allow use of a beam spot as large as several square centimeters, thus contributing to increase of the productivity.
However, to process a large-area substrate with an ordinary square or rectangular beam, it is necessary to move the beam in the two orthogonal directions. This is an item to be improved to increase the productivity.
This item can be greatly improved by deforming a beam into a linear shape that is longer than the width of a substrate to be processed and scanning the substrate with the beam relatively. (The scanning is performed by moving a linear laser beam in small steps with overlaps.) Details are described in Japanese Unexamined Patent Publication No. 5-112355.
A crystalline silicon film having a higher degree of crystallinity can be obtained by performing thermal annealing before the optical annealing. As for the method of thermal annealing, as disclosed in Japanese Unexamined Patent Publication No. 6-244104, a crystalline silicon film can be obtained at a lower temperature and in a shorter time than the case of using ordinary thermal annealing by utilizing the fact that such elements as nickel, iron, cobalt, platinum, palladium (hereinafter referred to as "crystallization catalyst elements" or simply "catalyst elements") have an effect of accelerating crystallization of amorphous silicon.
TFTs were formed in matrix form by using a crystalline silicon film formed by a conventional method in which an amorphous silicon film was formed on a glass substrate, annealed, and then subjected to laser annealing with a linear laser beam, and a distribution of their threshold voltages in the substrate surface was examined, which is shown in FIG. 2. It is seen from FIG. 2 that the distribution assumes a U-shape.
FIG. 4 shows an arrangement of TFTs on a glass substrate. In FIG. 4, TFTs are arranged in a matrix of 400.times.300 in an area of 40 mm.times.50 mm of a 100 mm.times.100 mm Corning 7059 substrate. In the data of FIG. 2, the horizontal axis shows positions of 400 TFTs on a horizontal full row (enclosed by a broken line in FIG. 4) of the substrate at the center in the vertical direction.
If TFTs of a pixel matrix that constitutes a pixel area of a liquid crystal display has the distribution of threshold voltages as shown in FIG. 2, there may occur display unevenness or an image defect.
An investigation into the cause of the above U-shaped distribution of threshold voltages in the substrate surface has revealed that it is very similar to a warp in a substrate immediately before application of laser light.
It has also been found that a glass substrate does not have a warp immediately after formation of an amorphous silicon film thereon and a warp occurs due to the fact that the silicon film contracts more than the glass substrate when the substrate is cooled after a heat treatment for crystallizing the amorphous silicon film by solid-phase growth. The warp occurs so as to be concave toward the film forming surface of a substrate.
FIG. 3 shows how laser annealing is performed with linear laser light 2 on a silicon substrate formed on a warped glass substrate 1. In FIG. 3, if the warped substrate 1 is subjected to laser annealing, the substrate surface deviates from focuses 3 of laser light differently at respective positions. It is considered that these deviations cause the silicon film to have different degrees of crystallinity in the substrate surface, so that threshold voltages exhibit a particular distribution in the substrate surface.
In the substrate with which the data of FIG. 2 was obtained, the overall depth of the U shape of the warped substrate was about 50 .mu.m immediately before the application of laser light.
The degree of warp depends on the temperature and time of the heat treatment, the substrate material, and other factors. In the case of a 100 mm.times.100 mm substrate, the depth of the U shape generally fell within the range of 20 to 200 .mu.m.
A concave warp occurred not only after the thermal crystallization of an amorphous silicon film formed on a glass substrate but also after slow cooling that was performed after an amorphous silicon film was subjected to laser annealing while being heated.